In ceramic honeycomb structural bodies widely used as a catalyst carrier and the like for emission control of exhaust gas, there is an increased need for improved emission control performance in order to comply with the emission regulations that have been tightened year by year, while reducing the pressure loss for satisfying the requests for fuel economy and high output.
In such circumstances, the growing trend is to increase the porosity at an opening end of cells of the honeycomb structure by further reducing the thickness of partition walls of the honeycomb structure, thereby reducing the pressure loss, as well as to increase the emission control performance by ensuring immediate activation of the catalyst after startup of the engine by reducing the heat capacity of the partition walls.
On the other hand, the development of such honeycomb structure having thinner walls raised the new problem that a variety of contaminants having entered the exhaust gas collide with the partition walls located at opening ends of cells of the honeycomb structure to damage the partition wall. This phenomenon is known as a erosion phenomenon.
To address this problem, already proposed is a honeycomb structure wherein a partition wall reinforcing part (reinforced partition wall part) having higher strength than the remaining part of the partition walls is provided in partition walls located at an opening end of cells (See Patent document 1, for example), and in this proposal, the manner of providing the partition wall reinforcing part is widely studied.
One conventionally known method of providing a partition wall reinforcing part includes the steps of: firing a base material having a honeycomb structure based on a cordierite forming raw material; adhering a slurry in which a cordierite forming raw material is dispersed in a dispersing medium, to the partition walls located at an opening end of cells of the base material; and drying and firing the resultant base material (See Patent document 1, for example).
However, in this method, the time-consuming firing process must be conducted twice, namely, firing of the base material and firing for providing the partition wall reinforcing part, so that there is still a great problem regarding the production efficiency, product cost and the like.
To address this problem, also suggested is a method in which a slurry prepared by dispersing a partition wall reinforcing material in a dispersing medium is caused to adhere to the partition walls located at an opening end of the cells before the base material having a honeycomb structure is fired, and then drying and firing were conducted. This method accomplishes firing of the base material and formation of the partition wall reinforcing part by a single firing step (See Patent document 1, for example).
However, as to this method, no substantial study about the difference in a material composition between the base material before firing and the base material after firing has been made at the moment. In particular, the base material before firing usually contains an organic binder or the like added for the purpose of, for example, improving the strength of the partition walls, however, no consideration was made on the fact that the organic binder is often a water-soluble compound such as methyl cellulose.
For this reason, if the step of forming a partition wall reinforcing part that is conventionally conducted after the firing step is carried out as it is before firing using a slurry prepared by dispersing the partition wall reinforcing material in water, the organic binder liquates out in the slurry to cause deformation in the partition walls and the like of the obtained honeycomb structure, making it unendurable to practical use due to reduction of isostatic strength or the like.
Additionally, in the case of the slurry prepared by dispersing a partition wall reinforcing material in a dispersing medium, dispersivity of the partition wall reinforcing material is likely to be insufficient because of sedimentation or aggregation of the partition wall reinforcing material caused by its physical property. This often results in variation or unevenness in the partition wall reinforcing part in degree of reinforcement. Accordingly, this production method suffers from the problem of impossibility to reliably obtain a ceramic honeycomb structure having uniform erosion resistance throughout the partition wall reinforcing part or the problem of increase in management burden for uniformly dispersing the partition wall reinforcing material.
To the contrary, the problem such as decrease in isostatic strength due to deformation of the partition wall or the like can be overcome by using a slurry prepared by dispersing the partition wall reinforcing material in a non-aqueous dispersing medium.
However, the problem of impossibility to reliably obtain a ceramic honeycomb structure having uniform erosion resistance throughout the partition wall reinforcing part or the problem of increase in management burden for uniformly dispersing the partition wall reinforcing material is not solved at all in this production method.
(Patent Document 1)
Japanese Unexamined Patent Publication JP-A 2000-51710
The present invention was devised in consideration of the above problems of the prior art, and it is an object of the invention to provide a ceramic honeycomb structure having uniform and excellent erosion resistance throughout the reinforced partition wall part, and a method of producing a ceramic honeycomb structure capable of obtaining a honeycomb structure having desired performance without causing deformation or the like in the partition walls while dramatically improving the productivity and the product cost, as well as forming a sophisticated and uniform reinforced partition wall part with high accuracy.